Metal hydride nanoparticles

ABSTRACT

A nanoparticle of a decomposition product of a transition metal aluminum hydride compound, a transition metal borohydride compound, or a transition metal gallium hydride compound. A process of: reacting a transition metal salt with an aluminum hydride compound, a borohydride compound, or a gallium hydride compound to produce one or more of the nanoparticles. The reaction occurs in solution while being sonicated at a temperature at which the metal hydride compound decomposes. A process of: reacting a nanoparticle with a compound containing at least two hydroxyl groups to form a coating having multi-dentate metal-alkoxides.

This application is a divisional application of allowed U.S. patentapplication Ser. No. 12/323,617, filed on Nov. 26, 2008, which claimsthe benefit of U.S. Provisional Patent Application No. 60/990,004, filedon Nov. 26, 2007. The provisional application and all other publicationsand patent documents referenced throughout the provisional applicationand this nonprovisional application are incorporated herein byreference.

TECHNICAL FIELD

The disclosed materials and methods are generally related to metal andmetal-hydride nanoparticles.

DESCRIPTION OF RELATED ART

Generally the currently used materials for metalizing of energeticformulations are micron scale aluminum particles. The main problemsassociated with the burn properties of the traditional metal additivesin energetic formulations have to do with the burn kinetics/speed beingimpeded by the aluminum oxide coating of the particles which arisesnaturally from the materials being exposed to air. The nanoscale Almaterials with various passivators for energetic formulations have beenexplored on an experimental basis (Berry et al., “Synthesis andcharacterization of a nanophase zirconium powder” J. of Mat. Chem., 13,2388-2393 (2003); Jouet et al., “Surface Passivation of Bare AluminumNanoparticles Using Perfluoroalkyl Carboxylic Acids” Chem. of Mat., 17,2987-2996 (2005); Jouet et al., “Preparation and reactivity analysis ofnovel perfluoroalkyl coated aluminum nanocomposites” Mat. Sci. Technol.,22, 422-429 (2006)).

Larger scale synthesis of air and moisture sensitive metal nanoparticlesis a challenge, and poses an obstacle to investigating the physicalproperties of the nano-scale materials via traditional techniques thatexpose the materials to air. To obtain materials that do not oxidizewhen handled in air while still retaining their properties, the surfaceof the nanoparticles can be protected by a passivating agent. In orderto keep the intrinsic properties of the unpassivated material, it isdesirable to maximize the active metal content of the material whileminimizing the amount of passivator present on the nanoparticle surface.

BRIEF SUMMARY

Disclosed herein is a nanoparticle comprising a decomposition product ofa metal hydride compound. The metal hydride compound is a transitionmetal aluminum hydride compound, a transition metal borohydridecompound, or a transition metal gallium hydride compound.

Also disclosed herein is a process comprising: reacting a transitionmetal salt with an aluminum hydride compound, a borohydride compound, ora gallium hydride compound to produce one or more nanoparticlescomprising a decomposition product of the metal hydride compound. Thereaction occurs in solution while being sonicated at a temperature atwhich the metal hydride compound decomposes.

Also disclosed herein is a process comprising: reacting a nanoparticlewith a compound containing at least two hydroxyl groups to form acoating comprising multi-dentate metal-alkoxides. The nanoparticlecomprises a metal that reacts with hydroxyl groups to form themetal-alkoxides.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be readily obtainedby reference to the following Description of the Example Embodiments andthe accompanying drawings.

FIG. 1 shows SEM micrographs of material 4 (Example 4) showing theblurring that occurs at higher magnification (bottom) as compared tolower magnification (top)

FIG. 2 shows TEM micrographs of material 3 (Example 3). Top: largeparticle enveloped in metal-alkoxide; Center: a population of smallnanoparticles on carbon background; Bottom: lattice fringing observedfrom nanoparticle overhanging on edge of carbon grid.

FIG. 3 shows material 5 (Example 5) initially (top) and after 24 h indH₂O (bottom)

FIG. 4 shows ²⁷Al-NMR of material 2 (Example 2) annealed at 89° C. underdynamic vacuum overnight (top) vs. ²⁷Al-NMR material 5 (bottom).Asterisks denote spinning side-bands.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth in order to provide athorough understanding of the present disclosure. However, it will beapparent to one skilled in the art that the present subject matter maybe practiced in other embodiments that depart from these specificdetails. In other instances, detailed descriptions of well-known methodsand devices are omitted so as to not obscure the present disclosure withunnecessary detail.

Disclosed is a homogeneous solution-based method used to producewell-defined passivated air and moisture stable transition metalaluminum/boron/gallium hydride nanoparticle materials. The synthesis maybe accomplished via a multi-step process. A transition metal salt isreacted with an aluminum hydride compound, a borohydride compound, or agallium hydride compound. The reaction occurs at a temperature at whichthe resulting transition metal hydride compound decomposes. For example,ZrCl₄ or Zr(BH₄)₄ may be reacted with LiAlH₄ at room temperature.Zr(AlH₄)₄ is produced, which decomposes at room temperature. The metalhydride compounds can contain hydrogen-bridging bonds, which may breakduring decomposition. This results in the loss of some, but notnecessarily all of the hydrogen in the nanoparticles in the form ofhydrogen gas. The use of sonication in solution may cause nucleation ofthe decomposition products so that nanoparticles are formed.

Suitable transition metals in the transition metal salt include, but arenot limited to, zirconium, hafnium, titanium, vanadium, scandium,yttrium, niobium, chromium, tantalum, thorium, or uranium. Reaction of ahafnium salt with a borohydride may produce HfB₂ as a decompositionproduct, which has a particularly high volumetric heat of combustion.

The nanoparticles may be annealed after they are formed. Annealing canconvert amorphous material into crystalline material, and may drive offsome or all of the remaining hydrogen. It may also cause reaction ofsome or all of any unreacted salts remaining in the nanoparticles. Theannealing may be performed under vacuum at a temperature that is up tofrom about one third to one half the melting point (Kelvin) of thedecomposition product in order to drive the dehydrogenation reaction tocompletion and further nucleate and/or crystallize the particles.

In a next step, a coating is formed on the nanoparticle by reacting itwith a compound containing at least two hydroxyl groups to form acoating comprising multi-dentate metal-alkoxides. Suitable compoundsinclude, but are not limited to, glycerol, sorbitol, and a carbohydrate.

The coating may be in the form of a xerogel from heating the reactionfollowed by drying in a vacuum, which causes shrinkage of the gel andpossible entrapped solvent. The aluminum or other metal can act as acrosslinking site for formation of the coating, as aluminum can bind to6 oxygen atoms. The process may pull some aluminum or other metal atomsfrom the surface of the nanoparticle and into the coating.

A coating made from glycerol, a small molecule, may be relatively thicksince the growing gel coating is more permeable for addition of moreglycerol— a relatively small molecule The thicker coatings may bedesirable for gas permeable applications where the fraction of reactivemetal is less important. Larger compounds such as sorbitol may form athinner coating, which may be desirable for maximizing the reactivemetal content for applications such as energetic materials. Theresulting material may be air and moisture resistant/stable, and containupwards of >90% of active metal by mass, and may be used as a metalizingadditive for energetic formulations.

The same type of coating may also be formed on other types ofnanoparticles that contain a metal that reacts with hydroxyl groups toform a metal-alkoxides. Suitable nanoparticles may contain aluminum,boron, silicon, zirconium, or hafnium, for example.

The initial reaction to produce the zirconium aluminum hydride was viadecomposition of zirconium tetrahydroaluminate (Zr(AlH₄)₄) while exposedto ultrasound produced by a benchtop ultrasonic bath. The particles weresurface passivated using carbohydrates and were shown to be stable inair and partially stable in water. TEM imaging suggests the existence ofsmaller particles made of a Zr—Al alloy that range in size from 1.8 nmto 7.9 nm in diameter and are interspersed with larger particles thatrange from tens to hundreds of nanometers in diameter. It was also shownthat the carbohydrate-derived coating of the nanoparticles is present asan aluminum alkoxide gel surrounding the core particles.

Based on the initial characterization of materials 1-5 (Examples 1-5),these materials have been shown to contain mainly Zr and Al, and thepassivated versions of these materials are robust and are air andmoisture stable. Based on the elemental analysis results, in the bestcase, Zr and Al constitute more than 90% of material 5. It has also beenshown that by varying the size of the carbohydrate that was used as thepassivator it is possible to change the amount of passivator remainingon the surface of the nanoparticles. Based on the TEM images of thelarger particles, combined with EDS data, it is possible to suggest amodel for the structures that is cocoon-like. The cocoon shell consistsof interlinked multi-dentate metal-alkoxides that form a polymeric shell(aerogel) that surrounds the metal-hydride particle.

This method can produce particles of smaller size that are moisture andair stable, which will enable better burn properties. This method can beextended to other higher density transition metals, such as Hf, Ta, Th,and U, which would produce much higher density materials, giving themunitions that carry it greater momentum while retaining the sameburn/shock wave characteristics as the lighter metals currently used.

The following examples are given to illustrate specific applications.These specific examples are not intended to limit the scope of thedisclosure in this application.

General

All air and moisture sensitive manipulations were performed in a VacuumAtmospheres glove box under an atmosphere of helium or via traditionalSchlenk technique under an atmosphere of nitrogen. Dry diethyl ether(Et₂O) was purchased from Aldrich packaged under nitrogen in a SureSealbottle, and was used without further purification. Glycerol waspurchased from Aldrich and was vacuum distilled prior to use. Lithiumaluminum hydride (LiAlH₄) was purchased from Aldrich and was furtherpurified by dissolution in Et₂O, followed by vacuum filtration andremoval of volatiles in vacuo. Lithium borohydride (LiBH₄) and zirconium(IV) chloride (ZrCl₄) was purchased from Aldrich and used as provided.Zirconium borohydride (Zr(BH₄)₄) was prepared as previously reported inliterature (Reird et al. J. Electrochem. Soc., 104(1), 21 (1957)).Oxygen bomb calorimetry was performed using a Parr model 1341 OxygenBomb calorimeter with a model 1104 Oxygen Combustion Bomb. Microanalysiswas performed by Complete Analysis Laboratories, Inc. SEM imaging wasperformed using a LEO 1550. TEM imaging was performed using a JEOL2200FS, equipped with a Gatan Ultrascan charge coupled device (CCD)camera. ²⁷Al—NMR was performed on a Bruker DMX500 at 11.7 T.

Example 1 Synthesis of Material 1 from ZrCl₄ and LiAlH₄

From a 200 mL Schlenk flask a clear solution containing 665 mg of LiAlH₄(LAH) in 80 mL of Et₂O was transferred dropwise via canula over 2 h to around-bottom 300 mL storage flask fitted with teflon valve joint, whichcontained 1.02 g of ZrCl₄ suspended in 100 mL of Et₂O being sonicated inice-water in a VWR 50HT benchtop ultrasonic cleaning bath. Upon initialaddition a white precipitate appeared in solution, producing a whiteslurry. Once the addition was completed, the flask was closed with aKontes Teflon valve and allowed to sonicate overnight at ˜50° C.(ultrasonic bath operating temperature) producing a black slurry.Volatiles were then removed in vacuo and 1.81 g of solids was isolated.The material was placed into a Pyrex sublimator and heated to 340° C.under dynamic vacuum. 1.66 g of the product black powder 1 was isolated.

Example 2 Synthesis of Material 2 from Zr(BH₄)₄ and LAH

From a 200 mL Schlenk flask a clear solution containing 3.49 g of LAH in120 mL of Et₂O was transferred dropwise via canula over 2 h to around-bottom 500 mL storage flask fitted with teflon valve joint, whichcontained 3.46 g of Zr(BH₄)₄ dissolved in 100 mL of Et₂O and thereaction was performed in the same manner as synthesis of material 1.Following the initial reaction, the flask was then taken into aglovebox, and a black powder material was centrifuged out of the slurryusing a benchtop centrifuge inside the glovebox. The black powder wasthen washed three times with 80 mL portions of Et₂O by mixing the powderand the Et₂O using a Pasteur pipette and centrifuging the material backout of the slurry. The material was then placed into a vacuum bulb anddried at room temperature (RT) under dynamic vacuum overnight to ˜200torr, producing 5.25 g of black powder material 2.

Example 3 Passivation of Material 1 with Glycerol to Make Material 3

1.51 g of 1 was mixed with 5 mL of glycerol in a 100 mL Schlenk flaskwith a ground-glass stopper. It was then placed on a Schlenk line andthe flask was heated overnight in a paraffin oil bath to 125° C.,initially producing visible bubbling, which eventually subsided. Thematerial was then cooled to RT and washed with ethanol (EtOH) in air andspun down in a benchtop high-speed centrifuge. The product black powderwas dried under dynamic vacuum at 40° C. producing 1.67 g of productblack powder 3.

Example 4 Passivation of Material 2 with Glycerol to Make Material 4

1.03 g of 2 was placed in a 20 mL vial and mixed with 2 mL of glycerolby manual agitation. Upon mixing with the glycerol the material formed ablack suspension which was visibly gently frothing. The foaming subsidedafter 1 h, and the vial containing the slurry was heated to about ˜50°C. The reaction began to foam vigorously, which necessitated thetransfer of approximately half of the reaction to a second 20 mL vial.The vials were allowed to stand at ˜50° C. for two days after which theywere taken out of the glovebox, and portions of EtOH were added to themto wash away the glycerol. The material visibly reacted with the EtOHproducing bubbling. After three EtOH wash and centrifugation cycles thebubbling was no longer noticeable. 1.30 g of black powder material 5 wasrecovered after drying under dynamic vacuum overnight, and taken intothe glovebox for storage.

Example 5 Passivation of Material 2 with D-Sorbitol to Make Material 5

In the glovebox, 372 mg of powder 2 was mixed with ˜1.5 g of D-sorbitolpowder in a 50 mL beaker and gently heated on a hot-plate until meltingof the sorbitol was observed. The entirety of the contents of the beakerwas allowed to melt while the suspension was manually agitated to mix itthoroughly. The liquid was visibly frothing and was allowed to stand onthe hotplate overnight at ˜110° C. Immediately following, ˜2.5 g ofglycerol was added to the mixture and stirred in with a spatula. Theblack suspension was then allowed to cool to RT and taken out of theglovebox, after which 10 mL of EtOH was added and mixed into thesuspension. This produced mild but visible bubbling, indicating areaction of EtOH with the particles. The material was repeatedly washedwith 40 mL portions of EtOH and spun down in a benchtop centrifuge inorder to remove any remaining excess sorbitol and glycerol. The materialwas then dried at RT under dynamic vacuum overnight producing 433 mg ofproduct black powder 5.

Example 6 SEM

The particles that were observed by SEM showed a wide distribution ofsizes. The SEM images of materials 3, 4, and 5 were significantlylimited with blurring occurring at higher magnifications due to chargingon the surface of the particles, as would be expected from particlesthat are non-conductive (see FIG. 1).

Example 7 TEM

TEM images of sample 3 suggest that it is a material made up of amixture of particles that are heterogeneous in size, with the bulk of itmade up of larger particles with diameter on the order of hundred(s)nanometers, which contain a core with a diameter of about half the totalparticle's diameter that is made up of a zirconium and aluminum material(FIG. 2). These cores are encased in cocoon-like shells made up ofaluminum alkoxides. EDS performed on the central dark portion of thelarge particle seen on the top in FIG. 2 exhibited peaks for Al and Zralmost exclusively, while the surrounding material that appears to beamorphous exhibited peaks for Al, C, and O.

As seen in the center and bottom images of FIG. 2, there is TEM evidenceof much smaller nanoparticles that range from 1.8 nm to 7.4 nm indiameter. Crystal lattice fringing is observed for these nanoparticlesin the TEM images with d-spacings of 2.08, 2.10, 2.19, and 2.21 Å, whichdo not match any oxide or hydride phases of Al and Zr, however, thereare mixed Zr—Al alloy phases that have peaks in the observed range,suggesting that the small nanoparticles are probably Zr—Al alloys (JCPDSRef. #s 13-510, 16-75, 17-891). Any attempts to perform EDS on theseparticles were unsuccessful due to their small size.

Example 8 Microanalysis and Oxygen Bomb Calorimetry

The microanalysis results from materials 1-5 are shown in Table 1. Thepassivated materials to be analyzed were shipped under air, while theunpassivated materials were shipped under helium. The results for C, H,and N are reported as an average of two trials no more than 0.10%different from one another.

TABLE 1 Microanalysis results for materials 1-5 Material % C % H % N %Al % Zr R R/% C 1 2 7.03 2.01 0.59 46.42 39.24 4.71 0.67 3 13.94 1.900.58 40.27 34.02 9.29 0.67 4 10.95 2.59 0.096 5 4.35 1.27 trace 49.5841.91 2.89 0.66

The metal content for the samples was determined by atomic absorption(AA). Column R (remainder) in Table 1 gives the value of the remainderfrom 100%, presumed to be 0 content as supported by the constant ratioof R to % C for the passivated samples 3-5 (last column of Table 1).From these results it is apparent that materials 3, 4, and 5 arepassivated against short-term (several days) air degradation.Furthermore, material 5 was stored in air for 15 days prior to beingshipped for microanalysis, and it still exhibits the best mass ratio ofmetal to passivator (organic portion).

According to the CRC Handbook of Chemistry and Physics, the heats ofcombustion of Zr and Al metals in O₂ are 2.9 kcal/g and 7.4 kcal/g,respectively. Since carbohydrates have been used as passivating agentand when burned carbohydrates yield approximately 4 kcal/g, it wouldfollow that the nanoparticles should fall somewhere between 2.9 and 7.4kcal/g. The results from oxygen bomb calorimetry for materials 2-5 arereported in Table 2, with the reported ΔH_(obs) values being theobserved heat output, and not the actual. (The actual AH values were notobtained due to the material reacting with O₂ prior to ignition.)

TABLE 2 Oxygen bomb calorimetry data for materials 2-5 # ConditionsΔH_(obs) (kcal/g) 2 @ 30 atm O₂ 1.48 3 @ 30 atm O₂ 4.01 4 @ 30 atm O₂4.34 5 @ 30 atm O₂ 3.56 5 @ 40 atm O₂ 2.11 5 in paraffin wax @ 40 atm O₂2.01 5 aged 24 h in dH₂O @ 40 atm O₂ 4.61

From the calorimetry data it is evident that the unprotected particlesof material 2 are mostly oxidized before ignition occurs, as compared tothe protected materials 4 and 5, which were made from 2. Anotherinteresting observation is the difference in combustion energy outputobserved for material 5 when combusted at 30 versus 40 atm of O₂. Thesignificantly lower energy output at the higher pressure confirms thateven when passivated, the material reacts with O₂ prior to ignition.These previous calorimetry experiments were conducted in the presence ofa known amount of ethylene glycol or glycerin, however, another attemptwas made to obtain calorimetry data from material 5 by embedding it intoparaffin wax; however, that yielded the same results as with ethyleneglycol, showing that paraffin is inadequate at protecting the particlesfrom 40 atm of O₂. Probably the most surprising result from calorimetrywas the energy output observed for material 5 after it was dispersed inwater for 24 h. The material was suspended in water and allowed to standovernight as an experiment to observe how it would settle out, as seenin FIG. 3. The observed heat output from the water-treated material 5was actually higher than for the same material before water treatment,suggesting that water reacts with the material's surface at leastpartially preventing O₂ from spontaneously reacting with particles. Thisis also supported by the observation of a slight evolution of gasbubbles when the particles were mixed with water. These results showthat material 5 contains at least 4.61 kcal/g.

Example 9 ²⁷Al Magic Angle Spinning NMR

From magic angle spinning experiments it was found that the unpassivatednanoparticle materials are electrically conductive, since they wereproducing eddy currents counteracting the instrument's magnetic fieldthereby resisting spinning. On the other hand, the passivated materialswere found to be non-conductive and had no problem spinning.

Initial magic angle spinning ²⁷Al-NMR experiments have shown that thereare two main peak regions for the Zr—Al nanoparticle materials, with theAl metal peak at 1646 ppm (relative to Al⁺³ in H₂O) and the Al non-metalpeaks have been observed in the 0-100 ppm region. As shown in FIG. 4,the unpassivated material exhibits a much smaller non-metal Al peak,whereas the passivated material has a significant non-metal Al peak.

Example 10 Synthesis of Hf—Al-Nanoparticle Material from Hf(BH₄)₄ andLAH

From a 200 mL Schlenk flask a clear solution containing 3.77 g of LAH in120 mL of Et₂O was transferred dropwise via canula over 2 h to around-bottom 500 mL storage flask fitted with teflon valve joint, whichcontained 5.90 g of Hf(BH₄)₄ dissolved in 100 mL of Et₂O while cooled to0° C. and the reaction was sonicated overnight producing a black slurry.Following the initial reaction, the flask was then taken into aglovebox, and a black powder material was centrifuged out of the slurryusing a benchtop centrifuge inside the glovebox. The black powder wasthen washed three times with 80 mL portions of Et₂O by mixing the powderand the Et₂O using a Pasteur pipette and centrifuging the material backout of the slurry. The material was then placed into a vacuum bulb anddried at 120° C. under dynamic vacuum overnight to ˜200 torr, producing7.08 g of black powder material.

Many modifications and variations are possible in light of the aboveteachings. It is therefore to be understood that the claimed subjectmatter may be practiced otherwise than as specifically described. Anyreference to claim elements in the singular, e.g., using the articles“a,” “an,” “the,” or “said” is not construed as limiting the element tothe singular.

What is claimed is:
 1. A process comprising: reacting a transition metal salt with an aluminum hydride compound, a borohydride compound, or a gallium hydride compound to produce one or more nanoparticles comprising a decomposition product of a metal hydride compound; wherein the metal hydride compound is a transition metal aluminum hydride compound, a transition metal borohydride compound, or a transition metal gallium hydride compound; and wherein the reaction occurs in solution while being sonicated at a temperature at which the metal hydride compound decomposes.
 2. The process of claim 1, wherein the transition metal is zirconium, hafnium, titanium, vanadium, scandium, yttrium, niobium, chromium, tantalum, thorium, or uranium.
 3. The process of claim 1, wherein the transition metal salt is ZrCl₄, Zr(BH₄)₄, or Hf(BH₄)₄ and the aluminum hydride compound is LiAlH₄.
 4. The process of claim 1, further comprising: annealing the nanoparticle.
 5. The process of claim 1, further comprising: reacting the nanoparticle with a compound containing at least two hydroxyl groups to form a coating comprising multi-dentate metal-alkoxides.
 6. The process of claim 5, wherein the coating is a xerogel.
 7. The process of claim 5, wherein the compound is glycerol, sorbitol, or a carbohydrate.
 8. A process comprising: reacting a nanoparticle with a compound containing at least two hydroxyl groups to form a coating comprising multi-dentate metal-alkoxides; wherein the nanoparticle comprises a metal that reacts with hydroxyl groups to form the metal-alkoxides.
 9. The process of claim 8, wherein the metal is aluminum, boron, silicon, zirconium, or hafnium.
 10. The process of claim 8, wherein the coating is a xerogel.
 11. The process of claim 8, wherein the compound is glycerol, sorbitol, or a carbohydrate. 